This invention relates to the preparation of N-(phosphonomethyl)glycine (“glyphosate”) from N-(phosphonomethyl)iminodiacetic acid (“PMIDA”), and more particularly to methods for control of the conversion of PMIDA, for the identification of reaction end points relating to PMIDA conversion and the preparation of glyphosate products having controlled PMIDA content.
N-(phosphonomethyl)glycine, known in the agricultural chemical art as glyphosate, is a highly effective and commercially important broad spectrum phytotoxicant useful in controlling the growth of germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants. Glyphosate is used as a post-emergent herbicide to control the growth of a wide variety of annual and perennial grass and broadleaf weed species in cultivated crop lands, including cotton production, and is the active ingredient in the ROUNDUP family of herbicides available from Monsanto Company (Saint Louis, Mo.).
Glyphosate and salts thereof are conveniently applied in aqueous herbicidal formulations, usually containing one or more surfactants, to the foliar tissues (i.e., the leaves or other photosynthesizing organs) of the target plant. After application, the glyphosate is absorbed by the foliar tissues and translocated throughout the plant. Glyphosate noncompetitively blocks an important biochemical pathway that is common to virtually all plants. More specifically, glyphosate inhibits the shikimic acid pathway that leads to the biosynthesis of aromatic amino acids. Glyphosate inhibits the conversion of phosphoenolpyruvic acid and 3-phosphoshikimic acid to 5-enolpyruvyl-3-phosphoshikimic acid by inhibiting the enzyme 5-enolpyruvyl-3-phosphoshikimic acid synthase (EPSP synthase or EPSPS) found in plants.
Various commercial processes are available for the preparation of glyphosate. For example, glyphosate may be produced by the catalytic oxidation of PMIDA in an aqueous medium. Such reaction may be conducted in either a batch or continuous mode in the presence of a catalyst that typically comprises particulate carbon, or a noble metal such as Pt on a carbon support. The catalyst is typically slurried in an aqueous solution of PMIDA within a stirred tank reactor, and molecular oxygen introduced into the reactor to serve as the oxidant. The reaction is exothermic. Temperature of the reactor is conventionally controlled by transfer of heat from the reaction mixture to a cooling fluid in an indirect heat exchanger. The heat exchanger may comprise coils immersed in the reaction mixture within the reactor, a jacket on the exterior of the reactor, or an external heat exchanger through which the reaction mixture is circulated from the reactor.
Recovery of the glyphosate product typically comprises one or more crystallization steps. The mother liquor stream or streams obtained in the crystallization may be recycled to crystallization or reaction steps of the process. A fraction of the mother liquor(s) is generally removed from the process in order to purge by-products. Crystallized glyphosate may be dried and sold as a solid crystalline product. A substantial fraction of the glyphosate crystals are commonly neutralized with a base such as isopropylamine, KOH, etc. in an aqueous medium to produce a concentrated salt solution. A concentrated formulation comprising such glyphosate salt solution, and often also other components such as, for example, various surfactants, is a principal product of commerce.
It is desirable to achieve substantially complete conversion of PMIDA to glyphosate during the course of the reaction. Although some unreacted PMIDA can be recovered and recycled to the reaction system, there are unavoidable losses that translate into loss of yield. The quality of the glyphosate product may also be compromised by residual PMIDA that is not removed in the glyphosate product recovery system.
Processes have been proposed by which the reaction can be deliberately conducted only to partial conversion, and the resulting relatively large fraction of unreacted PMIDA separated from the reaction mixture and recycled to the reaction system. However, processes which require the recycle of a high fraction of PMIDA involve capital intensive recovery and recycle systems, and require relatively complicated schemes for removal of impurities. As a consequence, it is often preferred to conduct the oxidation reaction to a high conversion, in some instances to a substantial extinction of PMIDA.
However, it is not desirable to extend the reaction time so as to unnecessarily expose the product glyphosate to the acidic and oxidative conditions of the aqueous reaction system. Glyphosate itself is subject to oxidation to form the by-product aminomethylphosphonic acid (“AMPA”). Relatively extended and/or severe reaction conditions can be effective to drive the conversion of PMIDA to glyphosate, but can also cause a loss of glyphosate yield by further conversion of glyphosate to AMPA. Extending the reaction cycle also increases the potential for loss of yield in the formation of N-methylglyphosate (“NMG”) by reaction of by-product formaldehyde or formaldehyde and formic acid with glyphosate. Other impurities such as N-formylglyphosate (“NFG”), N-methylaminomethylphosphonic acid (“MAMPA”) and glycine may also be formed. All these impurities and by-products may also potentially compromise the quality of the glyphosate product.
Consequently, there is a need in the art for methods for monitoring the conversion of PMIDA to glyphosate, and more particularly for identifying an end point at which, or residence time over which, a target conversion (target residual PMIDA concentration) has been attained. A PMIDA content up to about 6000 ppm by weight, basis glyphosate in the ultimate glyphosate product is typical of commercial production. In a product recovery process comprising crystallization of glyphosate such as that described, e.g., in U.S. Application Publication No. US 2005/0059840 A1, expressly incorporated herein by reference, the PMIDA content of the glyphosate product can be maintained at less than 6000 ppm if the PMIDA content of the product reaction solution is not greater than about 2500 ppm on a glyphosate basis.